Generally speaking, a method for virtualizing a logical storage area of multiple storage apparatuses in a storage system as a single storage resource is known. In a storage system such as this, a virtual logical storage space built using multiple storage apparatuses is referred to as a storage cloud.
Furthermore, in general, various types of physical storage apparatuses (a magnetic disk and flash memory) having a variety of attributes exists as storage apparatuses in a storage system. In the storage system, high-performance, high-reliability, high-cost storage apparatuses are used together with low-performance, low-reliability, low-cost storage apparatuses in accordance with the application. Specifically, frequently used data is stored in a physical storage area of a high-cost storage apparatus, and data archives and other such infrequently used data is stored in the physical storage area of a low-cost storage apparatus.
In the storage cloud, a logical storage space can be hierarchized using multiple logical units (LU) based on multiple storage apparatuses having various attributes. In the storage cloud, data based on an IO request (either a read request or a write request) from a host computer is stored in any of multiple LUs, and data is provided in accordance with the IO request from the host computer. In the storage cloud, in which the logical storage space is hierarchized, the data inside a LU comprising this logical storage space (hereinafter, referred to as volume data) is migrated to another LU in accordance with how often it is accessed.